Photonic integrated circuit edge coupler structure with reduced reflection for integrated laser diodes

ABSTRACT

A photonic chip, an edge coupler for an integrated photonic system and a method for coupling a laser to the photonic chip. The edge coupler includes a waveguide of the photonic system having a longitudinal axis. The longitudinal axis of a waveguide of the photonic chip is aligned with a longitudinal axis of the laser. The facet of the waveguide facing the laser is at a non-perpendicular angle with respect to the longitudinal axis. Light is transmitted from the laser into the waveguide via the angled facet.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority of U.S. ProvisionalApplication No. 62/531,414 filed Jul. 12, 2017, the disclosure of whichis incorporated herein by reference in its entirety.

INTRODUCTION

The subject disclosure relates to a method and apparatus for of couplinga laser beam to a photonic chip and, in particular, to reducingback-reflection at a coupling between the laser beam and the photonicchip.

A chip-based LIDAR (Light Detection and Ranging) system includes aphotonic chip that uses a laser to generate light. Light from the laserenters into the photonic chip in order to pass through various systemsof the photonic chip. An edge coupler is used to receive the light fromthe laser. However, light can be back-reflected at the edge coupler,which can degrade the linewidth and single frequency operation of thelaser. In addition, such back-reflection reduces the intensity of lightusable for LIDAR purposes. Accordingly, it is desirable to provide anedge coupler for the photonic chip that reduces an amount ofback-reflection that can be coupled into the laser cavity.

SUMMARY

In one exemplary embodiment, an edge coupler for an integrated photonicsystem is disclosed. The edge coupler includes a waveguide of thephotonic system having a longitudinal axis, and a facet at an end of thewaveguide for optical communication with a laser. The facet is at anon-perpendicular angle with respect to the longitudinal axis.

In addition to one or more of the features described herein, thewaveguide further includes an outer waveguide surrounding an innerwaveguide, and the light entering the outer waveguide via the facet istransmitted via the inner waveguide. The inner waveguide includes afacet located a selected distance from the facet of the outer waveguide.The facet of the inner waveguide is perpendicular to the longitudinalaxis. An end of the inner waveguide having the facet tapers to reducethe width of the inner waveguide in a direction approaching the facet.In various embodiments, the inner waveguide is made of Silicon and theouter waveguide is made of SiON. The angle of the facet of the waveguideis selected to optimize light coupling between the laser and thewaveguide and to minimize back reflection into the laser. A longitudinalaxis of a laser is made collinear with the longitudinal axis of thewaveguide.

In another exemplary embodiment, a method for coupling a laser to aphotonic chip is disclosed. The method includes aligning a longitudinalaxis of a waveguide of the photonic chip with a longitudinal axis of thelaser, wherein a facet of the waveguide facing the laser is at anon-perpendicular angle with respect to the longitudinal axis, andtransmitting light from the laser into the waveguide via the angledfacet.

In addition to one or more of the features described herein, thewaveguide further includes an outer waveguide surrounding an innerwaveguide, further comprising transmitting the light to enter the outerwaveguide via the facet, wherein the light is transmitted to thephotonic chip via the inner waveguide. The inner waveguide includes afacet located at a selected distance from the facet of the outerwaveguide. The facet of the inner waveguide is perpendicular to thelongitudinal axis. A section of the inner waveguide proximate the facetof the inner waveguide tapers to reduce the width of the inner waveguidein a direction approaching the facet. In various embodiments, the innerwaveguide is made of Silicon and the outer waveguide is made of SiON.The angle optimizes light coupling between the laser and the waveguideand minimizes back reflection into the laser. A longitudinal axis of thelaser is made collinear with the longitudinal axis of the waveguide.

In yet another exemplary embodiment, a photonic chip is disclosed. Thephotonic chip includes a waveguide having a longitudinal axis, and afacet at an end of the waveguide for optical communication with a laser.The facet is at a non-perpendicular angle with respect to thelongitudinal axis.

In addition to one or more of the features described herein, thewaveguide further includes an outer waveguide surrounding an innerwaveguide, wherein light enters the outer waveguide via the facet istransmitted via the inner waveguide. The inner waveguide includes afacet located a selected distance from the facet of the outer waveguide,the facet of the inner waveguide being perpendicular to the longitudinalaxis. The inner waveguide includes a section proximate the facet of theinner waveguide that tapers to reduce the width of the inner waveguidein a direction approaching the facet. The angle is selected to optimizelight coupling between the laser and the waveguide and to minimize backreflection into the laser.

The above features and advantages, and other features and advantages ofthe disclosure are readily apparent from the following detaileddescription when taken in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features, advantages and details appear, by way of example only,in the following detailed description, the detailed descriptionreferring to the drawings in which:

FIG. 1 shows a schematic diagram of a LIDAR system;

FIG. 2 shows an exemplary photonic chip suitable for use in the LIDARsystem of FIG. 1; and

FIG. 3 shows an arrangement of a laser and an edge coupler formaximizing an amount of light transmitted from the laser into the edgecoupler while reducing or minimizing an amount of light backscatteredinto the laser.

DETAILED DESCRIPTION

The following description is merely exemplary in nature and is notintended to limit the present disclosure, its application or uses. Itshould be understood that throughout the drawings, correspondingreference numerals indicate like or corresponding parts and features.

In accordance with an exemplary embodiment, FIG. 1 shows a schematicdiagram of a LIDAR system 100. The LIDAR system 100 includes a photonicchip 102, an optical coupler 104, and a microelectromechanical system(MEMS) scanner 106. A processor 108 controls operation of the photonicchip 102 in order to perform operations of the LIDAR system 100. Invarious embodiments, the LIDAR system 100 are disposed on asemiconductor integrated chip residing on a printed circuit board. Asdiscussed in further detail with respect to FIG. 2, the photonic chip102 includes a light source, such as a laser, an optical waveguidingnetwork and a set of photodiodes. The laser generates a transmittedlight beam 115 that is transmitted toward an object 110. Reflected lightbeam 117, which is due to interaction of the object 110 is opticallymixed with a fraction (<10%) of the transmitted light beam 115, in a setof photodiodes. The processor 108 controls the operation of the lightsource by controlling a waveform that modulates the light source. Theprocessor 108 further receives data from the photodiodes and determinesvarious parameters of an object 110 from the data.

In operation, the processor 108 controls the light source of thephotonic chip 102 to generate a modulated transmitted beam of light 115.The transmitted beam of light 115 passes through the optical coupler 104which collimates the transmitted light beam 115 and directs thetransmitted light beam 115 toward the MEMS scanner 106. The MEMS scanner106 steers the transmitted light beam 115 over a range of angles into asurrounding region of the LIDAR system 100.

The MEMS scanner 106 includes a vibrating member such as a vibratingmirror. The processor 108 controls an oscillation of the vibratingmember in order to steer the transmitted light beam 115 over a selectedrange of angles. In various embodiments, the MEMS scanner 106 is atwo-dimensional (2D) MEMS scanner, and the processor 108 controlsoscillation of the vibration member in two angular direction, such asazimuth and elevation.

Reflected light beam 117 is formed when object 110 interacts with thetransmitted light beam 115. A portion of the reflected light beam 117 isreceived at the MEMS scanner 106. The MEMS scanner 106 directs thereflected light beam 117 into the optical coupler 104 which focuses thereflected light beam 117 into the photonic chip 102.

In various embodiments, the LIDAR 100 system can be associated with avehicle and the object 110 can be any object external to the vehicle,such as another vehicle, a pedestrian, a telephone pole, etc. The LIDARsystem 100 determines parameters such as range, and Doppler as afunction of the azimuth and elevation of the object 110 and the vehicleuses these parameters to navigate with respect to the object 110 for thepurposes of avoiding contact with the object 110.

FIG. 2 shows an exemplary photonic chip 102 suitable for use in theLIDAR system 100 of FIG. 1. In various embodiments, the photonic chip102 is a scanning frequency modulated continuous wave (FMCW) LIDAR chip.The photonic chip 102 can be a silicon photonic chip in variousembodiments. The photonic chip 102 receives light from a coherent lightsource such as a laser 202. The laser 202 can be independent from thephotonic chip 102 or can be an integrated component of the photonic chip102. The laser 202 can be any single frequency laser which can befrequency modulated. In an embodiment, the laser 202 is a distributedBragg reflector laser. In various embodiments, the laser 202 generateslight at a frequency of 1550 nanometers (nm) or other wavelengthconsidered safe to human eyes. The laser 202 is coupled to a transmitterwaveguide 204 via an edge coupler 230 that receives the light from thelaser 202. The transmitter waveguide 204 directs the light from thelaser 202 out of the photonic chip 102 via a transmitter beam edge orgrating coupler 220 as transmitted light beam 115.

A local oscillator (LO) waveguide 206 is optically coupled to thetransmitter waveguide 204 via a directional coupler/splitter or amulti-mode interference coupler/splitter 210 located between the lightsource 202 and the edge coupler 220. The directional coupler/splitter orthe multi-mode interference coupler/splitter 210 splits the light fromthe laser 202 into a transmitted light beam 115 that continues topropagate in the transmitter waveguide 204 and a local oscillator beamthat propagates in the local oscillator waveguide 206. In variousembodiments, a splitting ratio can be 90% for the transmitter beam and10% for the local oscillator beam. The local oscillator beam is directedtoward a dual-balanced photodetector 214 that performs beammeasurements.

Incoming or reflected light beam 117 enters the receiver waveguide 208via a receiver beam edge or grating coupler 222. The receiver waveguide208 directs the reflected light beam 117 from the receiver beam edge orgrating coupler 222 to the dual-balanced photodetector 214. The receiverwaveguide 208 is optically coupled to the local oscillator waveguide 206at a directional coupler/combiner or multi-mode interferencecoupler/combiner 212 located between the edge or grating coupler 222 andthe photodetector 214. The local oscillator beam and the reflected lightbeam 117 therefore interact with each other at the directionalcoupler/combiner or multi-mode interference coupler/combiner 212 beforebeing received at the dual-balanced photodetector 214. In variousembodiments, the transmitter waveguide 204, local oscillator waveguide206 and receiver waveguide 208 can be optical fibers.

The dual-balanced photodetector 214 detects the frequency differencebetween in the transmitted light beam 115 and the reflected light beam117 due to reflection of the transmitter beam off of object 110, FIG. 1.The dual-balanced photodetector 214 is coupled to processor 108, FIG. 1.The processor 108, FIG. 1 determines, from the frequency differences,parameters of the object 110, such as range or distance, a direction ofarrival of the object 110, and a velocity of the object 110 relative tothe LIDAR system 100.

FIG. 3 shows an arrangement 300 of a laser 202 and an edge coupler 230for maximizing an amount of light transmitted from the laser 202 intothe edge coupler 230 while reducing or minimizing an amount of lightbackscattered into the laser 202. In various embodiments, the edgecoupler 230 is a cylindrical waveguide, such as an optical fiber. Theedge coupler 230 includes a longitudinal axis 306 and a facet 304 at anend of the edge coupler 230 facing the laser 202. A longitudinal axis320 of the laser 202 is aligned or substantially aligned with thelongitudinal axis 306 of the waveguide 230.

The edge coupler 230 includes an edge coupler waveguide 310 and aphotonic integrated circuit (PIC) waveguide 312 in an interior of anedge coupler waveguide 310. The edge coupler waveguide 310 therefore isan outer waveguide to the inner waveguide of the PIC waveguide 312. ThePIC waveguide 312 is made of a first material while the edge couplerwaveguide 310 is made of a second material. A longitudinal axis 315 ofthe PIC waveguide 312 is collinear or substantially collinear with thelongitudinal axis 306 of the edge coupler waveguide 310. The PICwaveguide 312 and the edge coupler waveguide 310 are optical waveguides.The edge coupler waveguide 310 includes a facet 304 at an end facing thelaser 202. The facet 304 is at an angle 0 that makes the facet 304non-perpendicular to the longitudinal axis 306 of the edge couplerwaveguide 310 and thus non-perpendicular to the longitudinal axis 320 ofthe laser 202 (i.e., facet 304 is non-parallel to facet 322 of laser202).

Having the facet of the edge coupler waveguide 310 at anon-perpendicular angle to the longitudinal 320 axis reduces an amountof back-reflection of light back into the laser 202. Also, when theangle 0 of the edge coupler facet 304 an optimum angle, a maximum lightcoupling occurs between the laser 202 and the edge coupler 230simultaneous with minimum back-reflection into the laser 202.

The PIC waveguide 312 includes a tapered end 316. The tapered end 316 isa selected distance from the facet 304 of the edge coupler waveguide310. The facet 314 of the PIC waveguide 312 is perpendicular to thelongitudinal axis and the tapered end 316 of the PIC waveguide 312tapers to reduce the diameter, radius or width of the PIC waveguide 312in the direction traveling toward the facet 314.

Any light that is reflected from the edge coupler waveguide 310 isdirected at the laser output facet 322 at an angle off of thelongitudinal axes 306, 320 and hence does not disturb the laseroperation. The PIC waveguide 312 captures the laser light and directsthe light to a location of the integrated circuit.

While the above disclosure has been described with reference toexemplary embodiments, it will be understood by those skilled in the artthat various changes may be made and equivalents may be substituted forelements thereof without departing from its scope. In addition, manymodifications may be made to adapt a particular situation or material tothe teachings of the disclosure without departing from the essentialscope thereof. Therefore, it is intended that the present disclosure notbe limited to the particular embodiments disclosed, but will include allembodiments falling within the scope thereof.

What is claimed is:
 1. An edge coupler for an integrated photonicsystem, comprising: a waveguide of the photonic system having alongitudinal axis; and a facet at an end of the waveguide for opticalcommunication with a laser, wherein the facet is at a non-perpendicularangle with respect to the longitudinal axis.
 2. The edge coupler ofclaim 1, wherein the waveguide further comprises an outer waveguidesurrounding an inner waveguide, wherein light entering the outerwaveguide via the facet is transmitted via the inner waveguide.
 3. Theedge coupler of claim 2, wherein the inner waveguide includes a facetlocated a selected distance from the facet of the outer waveguide. 4.The edge coupler of claim 3, wherein the facet of the inner waveguide isperpendicular to the longitudinal axis.
 5. The edge coupler of claim 3,wherein an end of the inner waveguide having the facet tapers to reducethe width of the inner waveguide in a direction approaching the facet.6. The edge coupler of claim 2, wherein the inner waveguide is made ofSilicon and the outer waveguide is made of SiON.
 7. The edge coupler ofclaim 1, wherein the angle optimizes light coupling between the laserand the waveguide and minimizes back reflection into the laser.
 8. Theedge coupler of claim 1, wherein a longitudinal axis of a laser iscollinear with the longitudinal axis of the waveguide.
 9. A method forcoupling a laser to a photonic chip, comprising: aligning a longitudinalaxis of a waveguide of the photonic chip with a longitudinal axis of thelaser, wherein a facet of the waveguide facing the laser is at anon-perpendicular angle with respect to the longitudinal axis; andtransmitting light from the laser into the waveguide via the angledfacet.
 10. The method of claim 9, wherein the waveguide furthercomprises an outer waveguide surrounding an inner waveguide, furthercomprising transmitting the light to enter the outer waveguide via thefacet, wherein the light is transmitted to the photonic chip via theinner waveguide.
 11. The method of claim 10, wherein the inner waveguideincludes a facet located at a selected distance from the facet of theouter waveguide.
 12. The method of claim 11, wherein the facet of theinner waveguide is perpendicular to the longitudinal axis.
 13. Themethod of claim 12, wherein a section of the inner waveguide proximatethe facet of the inner waveguide tapers to reduce the width of the innerwaveguide in a direction approaching the facet.
 14. The method of claim10, wherein the inner waveguide is made of Silicon and the outerwaveguide is made of SiON.
 15. The method of claim 9, wherein the angleoptimizes light coupling between the laser and the waveguide andminimizes back reflection into the laser.
 16. The method of claim 9,wherein a longitudinal axis of the laser is collinear with thelongitudinal axis of the waveguide.
 17. A photonic chip, comprising: awaveguide having a longitudinal axis; and a facet at an end of thewaveguide for optical communication with a laser, wherein the facet isat a non-perpendicular angle with respect to the longitudinal axis. 18.The photonic chip of claim 17, wherein the waveguide further comprisesan outer waveguide surrounding an inner waveguide, wherein light entersthe outer waveguide via the facet is transmitted via the innerwaveguide.
 19. The photonic chip of claim 18, wherein the innerwaveguide includes a facet located a selected distance from the facet ofthe outer waveguide, the facet of the inner waveguide beingperpendicular to the longitudinal axis, and wherein the inner waveguideincludes a section proximate the facet of the inner waveguide thattapers to reduce the width of the inner waveguide in a directionapproaching the facet.
 20. The photonic chip of claim 17, wherein theangle is selected to optimize light coupling between the laser and thewaveguide and to minimize back reflection into the laser.